In the art of die casting, a charge, or "shot" of material, such as molten metal, is forced into all the interstices of the die by means of a plunger. It has been found that the quality of the resulting casting depends to a great extent on the manner in which the acceleration and the velocity of the plunger varies during the operation. For a specific casting, it is often found, through a combination of therby and experiment, that a particular program of plunger acceleration and velocity produces a very good casting. Die casters frequently make a record of such a program, or velocity profile, as it is sometimes called, and attempt to repeat the profile for each subsequent casting. Accordingly, it is highly desirable to be able to vary and control the acceleration and velocity as desired and to repeat any profile found to be satisfactory.
The die casting plunger is frequently controlled by a means of a hydraulic ram, or shot cylinder, which in turn may be controlled by any of various systems. In one system, sometimes referred to as the "on-off" system, the shot cylinder is controlled by a four or five way main valve which in turn is controlled by an assembly of many adjustable valves which are individually adjusted and opened progressively to obtain the desired profile. Because of the many adjustments required, the set up of such an arrangement is quite time consuming. The system is not pressure compensated and, despite careful record keeping it has been found difficult to obtain the same velocity profile repeatedly.
Another system which has been used to control the shot cylinder is an electro-mechanical servo system using a multi-stage servo valve. An electric programmer generates a voltage which varies in accordance with the desired velocity throughout the cycle. This voltage is compared with voltages indicative of actual velocity and of output valve spool position to generate an error signal which is amplified and applied to the input of the servo valve. Such a system can establish an acceptable velocity profile but the electrical control equipment is complex and costly.
It is an object of the present invention to provide an improved control system capable of producing an acceptable velocity profile yet which is both inexpensive and reliable